Semiconductor devices are becoming smaller and more dense with the evolution of new technology. However, increases in circuit density produce a corresponding increase in overall chip packaging strategies in order to remain competitive. Chip and chip carrier manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating problems, reducing package size and weight, decreasing package costs, providing improved thermal efficiencies and better and more advanced chips. Whereas significant improvements are being made to eliminate systematic problem by reducing process variability. Process improvements alone are not sufficient to eliminate all the problems which effect both performance and reliability.
One way to increase performance and reliability would be to apply new materials to obtain chip fatigue life enhancement and/or to provide new environmental protection to the chip and/or the chip carrier or substrate. These advancements in the chip and chip carrier art have a profound effect on this industry.
Another way to increase performance and reliability is to provide the shortest and most efficient thermal cooling path for the integrated circuit chips. This could be done by bringing the chip physically as close as possible to the heat sink. Another way would be to provide more efficient cooling of the chip. However, when the chips are brought closer to the heat sink, means also have to be provided to securely provide a thermal contact between the chip and the heat sink. In some cases thermally conductive epoxies have been used to provide a better thermal contact between the chip and the heat sink, and in others some sort of thermal type paste has been used.
Research Disclosure, No. 270, Publication No. 27014 (October 1986), the disclosure of which is incorporated herein by reference, discloses a stick-on heat sink. A heat sink is attached to a module by sliding the module into the heat sink and where the edges of the heat sink snap close to secure the heat sink to the module. It is also disclosed that an adhesive or double sided tape could also be placed on the bottom surface of the heat sink to assure intimate contact between the module and the heat sink.
U.S. Pat. No. 4,092,697 (Spaight), the disclosure of which is incorporated herein by reference, discloses placing a film of thermally conductive material between the chip and the heat sink or heat radiator.
U.S. Pat. No. 4,233,645 (Balderes et al.), discloses placing a block of porous material which is impregnated with a suitable liquid between the chip and the heat sink to provide a thermally conductive path.
U.S. Pat. No. 4,849,856 (Funari et al.), the disclosure of which is incorporated herein by reference, discloses a direct chip to heat sink attachment process where a thermally conductive adhesive is used to directly secure the heat sink to the chip.
U.S. Pat. No. 4,939,570 (Bickford et al.), the disclosure of which is incorporated herein by reference, discloses another direct chip to heat sink attachment process where a thermally conductive adhesive is used to directly secure the heat sink to the chip.
U.S. Pat. No. 4,999,699 (Christie, et al.), the disclosure of which is incorporated herein by reference, discloses solder interconnection whereby the gap created by solder connections between a carrier substrate and semiconductor device is filled with a composition obtained from curing a preparation containing a cycloaliphatic polyepoxide and/or curable cyanate ester or prepolymer thereof; filler having a maximum particle size of 31 microns and being at least substantially free of alpha particle emissions.
U.S. Pat. No. 5,249,101 (Frey, et al.), the disclosure of which is incorporated herein by reference, discloses a coverless chip carrier which uses at least two encapsulants. The first encapsulant is used to provide flip-chip fatigue life enhancement. The second encapsulant is used to provide limited environmental protection. A third encapsulant is also required for carriers using peripheral leads to contain the second encapsulant prior to curing. Also disclosed is that the encapsulant have a CTE (Coefficient of Thermal Expansion) which is within 30 percent of the CTE of the solder balls.
The inventors of this invention, however, are using an entirely different approach to solve this age old problem of chip carrier cost, weight, and size. They have discovered that a single specific encapsulant material can be used to provide the full environmental protection, including solvent exposure, and fatigue enhancement.
With this single encapsulant one could also use a double-sided, pressure-sensitive, thermally-conductive adhesive tape to directly attach the chip to the heat sink and to provide a secure thermal contact between the two.
Furthermore, they have also discovered a novel method and structure which ensures the integrity of the bond between the heat sink and the substrate, while the chip connections and other features are protected by the novel encapsulant.
The structure and process of this invention offers several advantages over the prior art. For example, it provides a simplified modular construction, therefore, it utilizes fewer materials and process steps for assembly, and allows ease of workability or repair of the assembled module.